1. Field of the Invention
The present invention relates to a laminated coil with an excellent durability for motor, a method for manufacturing the coil, and further a motor using the coil.
2. Description of the Prior Art
A brushless motor generally comprises, as shown in FIG. 4, a rotating shaft 2, a rotor 4 composed of permanent magnets formed integral with this rotating shaft 2, and a stator 6 composed of a coil (armature coil) facing the rotor 4.
In this type of brushless motor, the coil is composed of a winding using a wire. Recently, however, there has been proposed such a method, thanks to improvements in printed wiring technology, for forming a coil sheet 14 by printing a conductor pattern for coil as a printed pattern on a film 10 produced of an .epoxy resin, polyester resin, polyimid resin, etc. and then, as shown in FIG. 5, forming the coil by laminating a number of said coil sheets 14. This method is disclosed in for example Japanese Patent Laid-Open No. SHO 57-68565, 57-186940, 57-68656, or 5833958.
The laminated coil as described above requires no winding of a metal wire on the core unlike prior-art coils, and winding density is high, accordingly, is advantageous for obtaining a small-type motor with windings of high winding density.
The coil of prior art described above, however, has such a problem that since it is required to laminate plural coil sheets 14 by the use of an adhesive, it is required a process for forming a uniform layer of adhesive on each sheet surface and attaching these coil sheets 14. As a result the cost of the coil is very high. Also, because the coil sheet, being made of a plastics or a composite material, has a low heat resistance, and moreover is liable to be affected by heat caused by an iron loss or a copper loss or by an ambient temperature, a motor operation becomes unstable so that using condition such as the ambient temperature is limited.
Therefore, in an attempt to solve the above-described problem of the prior-art laminated coil, there has been proposed such a laminated coil, in Japanese Patent Laid-Open No. SHO 64-59902, that is produced by forming a conductive pattern for coil on a green sheet obtainable from a ceramic powder, through a screen printing, photolithography, or plating technique, and by laminating such plural green sheets on which the conductive pattern for coil has been formed, and then by baking the thus laminated green sheets as one body.
However, a ceramic material, for example even alumina, to be used for such a multilayer substrate is generally too expensive to use for this type of small motors. Furthermore, when the above-described method of manufacturing is adopted, the melting point of the conductor for coil is required to be higher than that of the baking temperature for the ceramic material. For example, in the case of a combination of the alumina (baking temperature, 1600.degree. C.) and Au (melting point, 1063.degree. C.), Au melts in the ceramic, resulting in being small spheroids or evaporating.
Generally, there is such a tendency that the higher the melting point of a conductor, the greater the resistance of the conductor, and reversely the lower the melting point, the less the resistance. With a loss taken into consideration, of course, a metal of less conduction resistance is demanded for use as a conductor. Therefore, for a ceramic material of the green sheet, it is demanded the ceramic material which is baked at a lower temperature than the melting point of Au and Cu which are good conductors. This means that the laminated coil can be composed only of a further special, expensive ceramic material. Therefore, it should be noted that the technique disclosed in Japanese Patent Laid-Open No. SHO 64-59902 previously stated for laminating plural green sheets and baking them as one body is infeasible because of expensiveness of the laminated coil.